Save your updated AndroidManifest.xml file, rebuild your app, and deploy to a device or virtual device running Android 2.2 or above. Now if you open Settings > Applications, and select your app, you should be offered the option to "Move to SD card" :)

I've just updated my android games to offer users this option, and it'll be something I'll be doing as standard in all my future apps. If you're an app developer you should consider doing the same... As a user I appreciate being given the option to choose where to install my apps - and I'm sure others do too!